A Group III nitride semiconductor such as AlxGayIn1-x-yN (0≦x≦1, 0≦y≦1) including gallium nitride (GaN) is wide in forbidden band width, high in electron saturation speed, and high in breakdown field, compared with silicon (Si) and gallium arsenide (GaAs). Therefore, its research and development has been enthusiastically carried out in order to realize a high-frequency, high-power, and high-breakdown-voltage Group III nitride semiconductor field effect transistor (FET) made of a material such as AlGaN or GaN.
When a high voltage is applied between a gate electrode and a drain electrode (between a gate and a drain) while the Group III nitride semiconductor FET is operated, a drain current is reduced, a current leak is increased, and breakdown will be observed. These are caused by a current collapse. The current collapse occurs because a high electric field is applied at a gate electrode end, electrons are accelerated and become hot electrons, and as a result, the electrons are trapped to a level existing in the forbidden band of the Group III nitride semiconductor (refer to non-patent literature 1, for example). Because the electrons are trapped to that level, a channel of the FET becomes narrow, so that the drain current is reduced.
The level existing in the forbidden band of the Group III nitride semiconductor is attributed to a surface of the semiconductor, or attributed to a bulk semiconductor. As a method for reducing the level attributed to the surface of the semiconductor, a protective film made of SiN is used. More specifically, it is reported that the surface level can be reduced when the SiN protective film is used, so that the current collapse can be prevented (refer to a non-patent literature 2, for example). However, even when the SiN protective film is used, a high electric field is applied at the gate electrode end when the FET is operated at a high voltage, so that the current collapse cannot be sufficiently prevented from being generated.
Thus, as a means proposed for relaxing the electric field applied at the gate electrode end, a gate field plate electrode that is electrically connected to the gate electrode is disposed between the gate electrode and the drain electrode (refer to patent literature 1, for example).
However, there is a possibility that the gate field plate electrode causes an increase in parasitic capacity Cgd between the gate and the drain, and a sufficient gain cannot be obtained. Thus, according to a proposed method, a field plate electrode that is connected to the source electrode is disposed between the gate electrode and the drain electrode, so that Cgd is reduced, the current collapse is prevented, and the gain is increased (refer to patent literature 2, for example).